LONDON (Reuters) - As Olympic officials prepare a bigger
dope testing program than at any previous Games, researchers
say a common genetic variation may be helping some athletes to
beat testosterone doping tests and get away with cheating.

People with this change had normal testosterone levels even
after they were injected with high levels of the steroid, said
Jenny Schulze, a geneticist at Sweden's Karolinska Institute,
who worked on the study.

"Genetic factors may play an important role in the accuracy
and sensitivity of testosterone doping tests," Schulze said.
"This is of interest not only for combating steroid doping in
sports, but also for detecting and preventing steroids abuse in
society."

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Athletes have long used testosterone to gain a competitive
edge. The male hormone -- one of the most commonly used
anabolic steroids -- increases muscle size and strength as well
as helping to speed recovery from training or injury.

The World Anti-Doping Agency (WADA), which funded the
Swedish research, has sought to stamp out steroids amid a slew
of recent drug-related scandals in a range of sports.

U.S. cyclist Floyd Landis received a two-year ban after a
French laboratory turned up traces of synthetic testosterone in
a sample he provided during the 2006 Tour de France.

GATLIN BAN

Prosecutors in the United States plan to seek a new
criminal indictment of Major League Baseball's all-time home
run leader Barry Bonds stemming from an investigation into drug
use in sports.

Olympic 100 meters champion Justin Gatlin is appealing
against a four-year doping ban for testing positive for
testosterone in 2006.

The international drugs watchdog estimated that anabolic
steroids accounted for about 43 percent of positive results
among athletes in 2005 with testosterone, nandrolone and
stanozolol among the most prominent.

In the study, Schulze and her colleagues chose 55 men, of
whom 15 percent had the important change in the UGT2B17 gene
normally involved in the secretion of testosterone.

They injected each of them with a 360-milligram dose of
testosterone and checked them for signs of suspicious levels of
the male hormone over a 15-day period.

They found that the common variation could give rise to
wildly different results, even when the dose was the same. The
results were not skewed for people without the genetic change.

For example, six days after the injection when the
testosterone levels are highest in urine, about 59 percent of
people with the variation had a result officials would deem
suspicious. That figure was 100 percent for those without the
genetic advantage.

MOST COMMON

"Nearly half of the individuals in our study who carried
this genetic variation would go undetected in a regular doping
test after a single 360-milligram dose of testosterone,"
Schulze said.

"If you don't have the gene you don't secrete testosterone
in your urine."

The genetic change was most common in the East Asian
countries of Japan, Korea and China where about 65 percent of
people carry the variation, Schulze said in a telephone
interview.

For Caucasians and people of African descent the figure was
10 percent or less, she added.

The researchers plan to publish their findings in the
Journal of Clinical Endocrinology & Metabolism in June.

More research was needed but one idea would be for genetic
profiles of each athlete to be drawn up to determine what was
the appropriate cut-off point for testosterone levels, Schulze
said.

"When you talk about genetic variations, 10 percent is
still quite a high number," she said. "Current doping tests are
based on people who do not have the genetic variation."

Olympic authorities say they will carry out some 4,500
doping tests at August's Beijing Games, up from 3,500 at the
Athens Olympics four years ago.